This video excerpt from NOVA examines a genetic screening technique called preimplantation genetic diagnosis, or PGD. PGD enables researchers to identify mutations in an embryo’s DNA that can lead to birth defects and more than 300 diseases, including Huntington’s, cystic fibrosis, and some cancers. While the technique allows parents who use in vitro fertilization (IVF) to better understand the risk that their child might be diagnosed with a terrible disease, it is also being used in some parts of the world to select for sex. Serious ethical questions are being raised about this use and about the possibility of expanding the list of traits that are screened for.

This video is available in both English and Spanish audio, along with corresponding closed captions.

There is no known cure for many debilitating and progressive diseases, including cystic fibrosis, Huntington’s disease, and sickle cell anemia. What treatment there is provides little comfort and hope for the patient. But a screening technique called preimplantation genetic diagnosis (PGD) offers couples with a strong family history of some genetic diseases a way to have children with far less worry that they will pass on disease genes to the next generation.

PGD is a procedure that involves testing the genetic makeup of embryos produced through in vitro fertilization (IVF) before they are transferred into the uterus in the hopes of establishing a pregnancy. It is most often performed for women with a history of miscarriage to spot chromosomal abnormalities associated with the spontaneous loss of an embryo or fetus during pregnancy. Because hormonal treatments most often used in IVF produce multiple eggs in a woman’s ovaries, they typically generate several embryos. From these embryos, parents can select which ones to transfer. A single cell removed from each embryo in the first few days of development is analyzed for certain genetic markers. Depending on the disease or diseases being screened for, lab technicians may look for chromosome abnormalities, gene abnormalities, or both. If, for example, they are screening for Down syndrome, they will look for an extra chromosome 21. If they are screening for Huntington’s disease, they will look specifically at a single gene on chromosome 4 that is linked to that disease. Because only embryos without the mutations being tested for are used, couples do not have to wait many weeks into a pregnancy—when prenatal testing (such as amniocentesis) is normally done—to learn if the fetus is at risk for a heritable disease.

Few would argue with the main benefits PGD offers: namely, a way to spare women recurrent miscarriages and a way to have children who do not suffer from certain diseases and disorders. Still, the technique is controversial. Disability rights activists argue that its use could promote discrimination against the sick or disabled, with society viewing them as individuals who should never have been born. Other critics are concerned that PGD is increasingly being used to select for sex and less serious genetic anomalies, such as late-onset conditions. Still others worry that it will be used to select for certain cosmetic and behavioral traits, such as height, skin complexion, and intelligence.

To those critics, PGD proponents suggest that people should be able to decide for themselves what they wish to avoid. They also point out that the technique selects traits only from genes that the parents themselves carry, and that the number of embryos parents have to choose from is limited by the small number available during IVF. This factor alone, they argue, makes it extremely unlikely that a couple could “design” their child to have a perfect complement of desirable traits. This notion of “designer” children has drawn comparison with eugenics, an early 20th-century practice that sought to breed a genetically more “fit” human race. Some bioethicists contend that prospective parents seeking to control a child’s traits may regard their offspring as "projects" for them to perfect—not as unique individual human beings who must be accepted and helped to flourish.

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